CEN2

TgCEN2 is a calcium-binding centrin family protein that operates from four spatially distinct cytoskeletal compartments in Toxoplasma gondii — an apical pool, the basal end, the centrioles, and cortical annuli at the base of the apical cap — each of which is populated with different incorporation kinetics, allowing its multiple roles to be dissected through ordered, stepwise conditional depletion (PMID:31182647, PMID:31206964). The apical pool is specifically required for the egress-invasion cycle: its loss abolishes microneme secretion, gliding motility, host cell invasion, and egress (PMID:31206964). Depletion timing separates these functions, with early loss from the apical compartment producing an invasion deficiency and later loss from the remaining compartments producing intracellular replication defects, establishing that the protein performs distinct compartment-specific roles (PMID:31182647). Biophysically, TgCEN2 binds only a single Ca2+ ion with low affinity and, unlike TgCEN1, undergoes no significant Ca2+-dependent conformational change, hydrophobic patch exposure, or self-assembly, indicating it does not act as a classical Ca2+ sensor (PMID:32759683). How TgCEN2 mechanically supports secretion, invasion, and replication at each compartment beyond these observations has not been characterized in the available corpus.

1. 2019 High

   It was unknown how a single centrin could contribute to multiple cytoskeletal structures; resolving its localization and the kinetics of its turnover provided the framework for assigning function to individual compartments.

   Evidence Conditional knockdown with transcriptional and protein stability control combined with fluorescence localization in T. gondii

   PMID:31182647 PMID:31206964

   Open questions at the time

   • Molecular partners that recruit CEN2 to each of the four compartments are not identified
   • The structural basis of differential incorporation kinetics is not defined
2. 2019 High

   By exploiting the ordered depletion that empties the apical pool first, the apical CEN2 population was shown to be specifically required for the microneme secretion, motility, invasion, and egress program, distinguishing its lytic-cycle role from its other functions.

   Evidence Conditional protein depletion with microneme secretion, motility, invasion, and egress assays in T. gondii

   PMID:31206964

   Open questions at the time

   • The mechanism linking apical CEN2 to microneme exocytosis is unknown
   • Direct molecular effectors at the apical pool are not identified
3. 2019 High

   Correlating the sequence of phenotypes with compartment-specific loss demonstrated that CEN2 has genuinely distinct functions at different locations — invasion first, then replication — rather than a single global role.

   Evidence Conditional knockdown with invasion and replication assays correlated to sequential compartment depletion in T. gondii

   PMID:31182647

   Open questions at the time

   • How CEN2 supports replication at the centrioles/basal end mechanistically is unresolved
   • Whether the replication defect is a direct or downstream consequence of depletion is not established
4. 2020 Medium

   It was unclear whether TgCEN2 acts as a Ca2+ sensor like related centrins; in vitro biophysics showed it binds one Ca2+ with low affinity and lacks the Ca2+-triggered conformational and self-assembly behavior of TgCEN1, reframing it as a non-sensor calcium-binding protein.

   Evidence In vitro Ca2+ binding measurements, structural analyses, ANS hydrophobic-exposure and self-assembly assays comparing TgCEN2 and TgCEN1

   PMID:32759683

   Open questions at the time

   • No in-parasite functional validation of the Ca2+-binding properties
   • The functional consequence of low-affinity Ca2+ binding for its compartment roles is unknown
   • Single lab, in vitro only

• The molecular partners and mechanistic effectors through which CEN2 drives microneme secretion, invasion, and replication at each of its four compartments remain undefined.
• No interacting proteins mapped to any compartment
• No structural model of CEN2 in situ
• Mechanism connecting a non-sensor calcium-binding protein to secretion and invasion is unknown